The field of the invention is the radio tracking of remotely located devices. Specifically, this invention relates to channel-acquisition and channel-access methods used by large numbers of unsynchronized radio-transceivers, operating in a shared radio-band environment, to communicate with sensing and locating devices in a location-tracking network.
Personal safety has become a visible concern for many in society, particularly those who frequent public places such as college campuses. Many personal-safety devices have been proposed and marketed which seek to alleviate these concerns. One important requirement for all safety devices is to provide an alarm that allows authorities to provide a prompt response.
Once an alarm has been received, the overriding goal for authorities is to promptly determine the location of the emergency call. Once the person""s location is known, steps can be taken to provide appropriate aid in a reliable and timely manner. Additionally, any location determination system must instill users with enough confidence that attempts to use the system will be successful.
While it is a conceptually simple task to transmit a signal that can be tracked in an interference free environment, practical problems exist. For example, there is an increasing demand for the transmission spectrum by a myriad of types of wireless services, to the point where many bands must be shared to allow many wireless services to be offered. This sharing results in a complex communications environment, and many sophisticated techniques must be employed to ensure that systems will continue to operate reliably in spite of the xe2x80x9csignal degradationxe2x80x9d caused by the wireless environment. Signal interference problems also exist. For example, interference can originate from other parts of the same system, from other compatible systems, such as might occur due to nearby campuses operating the same kind of system at each campus, from incompatible systems such as cordless telephones or point-to-point data links that typically also use the shared-band, and from equipment operating at higher power levels outside of the band (spurious radiation).
A variety of access methods have been previously used in the data and wireless fields. For example, a multiple access method known as ALOHA, is well known. In this simple method, which is effective for access to a medium that has little demand, a transmission is made whenever the data is ready, and the success of the transmission is based on the receipt of a positive acknowledgment of receipt of the message. If the acknowledgment is negative, indicating that either the message was not received and there was no acknowledgment, or that the message was received corrupted, then the mechanism will re-try, but with a scheme for backing off by a random delay, to prevent the system from xe2x80x9cchokingxe2x80x9d. When media traffic increases, significant numbers of messages are corrupted by this method, and overall throughput falls to very low levels, with most of the time being spent on re-try attempts.
An improved method, Carrier Sense Multiple Access (CSMA), first senses whether a transmission is in progress before attempting to transmit, thereby preventing to large degree, the corruption of messages already in the process of transmission. However, this method can waste some of the system""s capacity when a medium number of transmissions are attempted since the algorithm waits a significant length of time before re-attempting transmission.
Another algorithm used for multiple access to a frequency channel is p-persistent CSMA. Two constants are used with this algorithm: T, the end-to-end propagation delay of the bus, and p, a specified probability. A station using the p-persistent algorithm senses the channel and then the following occurs. If the channel is sensed idle, a random number between zero and one is chosen. If the selected number is less than p, the packet is transmitted; if not, the station waits T seconds and repeats the complete algorithm (which includes the contingency that the channel may be busy). Also, if the channel is busy, the station persists in sensing the channel until it is found to be idle and then proceeds as described above.
The p-persistent algorithm also uses the sense information to avoid transmitting when the channel is busy. It differs from the nonpersistent algorithm (which never persists in transmitting since it always backs off), by persisting in attempting to transmit. When the channel becomes free, this fact is immediately sensed by a station using the p-persistent algorithm. To avoid collisions with other ready stations that are employing the same strategy, or at least to inject flexibility into the algorithm, a station transmits only with probability p when the channel becomes free.
The delay T is chosen so that two stations sensing the channel to be free at the same time will not collide if one transmits and the other delays. In time T the leading edge of the transmission from the transmitting station will reach the sensor of the second station and the sensed signal will prevent it from transmitting. The parameter p can be chosen to optimize the algorithm for the application and its message traffic patterns.
It is an object of the invention to provide a low interference-potential mechanism for multiple access by unsynchronized locator-transceivers using the frequency hopping techniques required for shared band-use.
It is a further object of the invention to significantly reduce effects of interference while operating in the ISM band, to enhance communication reliability.
It is still another object of the invention to provide a reliable spread-spectrum signaling technique for tracking the location of the locator-transceiver.
It is yet another object of the invention to provide a simple mechanism for prioritizing access of different classes of transmission.
It is a further object of the invention to provide a tracking system that is quickly and reliably locates people using the radio-transceiver.
It is another object of the invention to provide a device that allows security personnel to quickly come to the aid of people who experience a threat to their personal safety anywhere in or near a campus or similar environment.
It is a further object of the invention to provide a device which, when activated, transmits an identification signal whose source location can be remotely determined and tracked with enough precision for security personnel to quickly come to the aid of the person who activated it.
It is still another object of the invention to provide a system that is reliable and offers an on-demand confidence-test feature.
It is yet another object of the invention to provide a system that is both robust and user-friendly.
It is another object of this invention to effectively minimize the creation of interference in the shared band in which it operates, thereby ensuring communications-reliability, while still maximizing the throughput of the system.
It is another object of this invention to effectively minimize the deleterious effects of interference inherent in the shared band in which it operates, thereby ensuring communications-reliability,, while still maximizing the throughput of the system.
It is still another object of the present invention to provide a system for locating an alarm call which uses of a number of different wireless locating technologies.
It is still another object of the invention to provide a system which uses shared radio-communication bands.
These and other objects of the invention are provided by a system for determining the location of a device comprising means for transmitting a location determination transmission over a communication channel; a plurality of detection device for receiving said location determination transmission; and means for prioritizing access to said communications channel including means for having said remote device wait to transmit over said communication channel for a predetermined period.